To be honest I wouldn't mind any character from before Gen5. But to be more specific including the ones you mentioned I would also like to see Paul and Cynthia. You already probably know how much I love Cynthia, but reason I would like to see Paul return was cause of the chemistry between him and Ash, and I would love to see to they would act with each other after this far into the series after he began to respect Ash.

It's like "take sand to the beach" say it, as almost every pokémon fan want it, but... I think Brock must come back.
And I think it could happen, since he already left and returned to the show at other times.

Among those there are no possibilities, I'd like the return of Iris, because as I said here she is my favorite female character on the anime.